1. Field of the Invention
The present invention relates to an optical pickup device for reading data from, and writing data into, a recording medium such as an optical disk and a card, and more particularly to a multi-beam optical pickup device capable of forming light spots on a plurality of tracks of a recording medium at the same time.
2. Description of the Related Art
In one of the methods of reading at the same time data recorded on a plurality of tracks of a recording medium such as an optical disk, light is emitted from an optical pickup device and focussed upon respective tracks of the recording medium, and light reflected from the tracks is detected with respective photodetectors. Methods of forming a plurality of light spots as illustrated in FIGS. 7 and 8 are known. Basics of these methods will be described with reference to FIGS. 7 and 8 in which elements similar to those of the embodiments to be described later are represented by using identical reference numerals. With the method illustrated in FIG. 7, a semiconductor laser array 60 having semiconductor lasers same in number as the number of necessary light spots is used to emit light from light sources 61a, 61b, 61c, and 61d of the semiconductor lasers. With the method illustrated in FIG. 8, a single semiconductor laser 10 is used. Light emitted from a real laser light source 11 (“real” is used to distinguish from “imaginary” laser light sources 12a, 12b, and 12c to be later described) of the semiconductor laser 10 is divided by a diffraction grating 64 into a plurality of light fluxes which serve as those emitted from the light sources 61a, 61b, 61c, and 61d of the semiconductor laser array 60 shown in FIG. 7.
The method illustrated in FIG. 7 using the semiconductor array 60 is, however, associated with the following problems: (a) since a plurality of semiconductor lasers are assembled in one package or chip, there is a limit of reducing the size of the semiconductor laser array; (b) the number of connection terminals increases; (c) it is difficult to make the array compact because a surface area thereof is required to be as large as it allows heat dissipation; and (d) manufacture yield and cost are bad because it is necessary to use a plurality of semiconductor lasers having uniform characteristics.
Although only a single semiconductor laser can be used with the diffraction grating 64 and manufacture cost can be lowered, it is necessary to mount the diffraction grating 64 at the position as near to the semiconductor laser as possible in order to make compact the optical pickup device. In this case, as shown in FIG. 9; the nearer to the semiconductor laser the diffraction grating is mounted, the larger the angle θ between the light beam incident upon the diffraction grating 64 from the real laser light source 11 and the diffraction light beam emitted from the diffraction grating 64 (θ1>θ2). Therefore, astigmatism and coma of a light spot become large, which make the diameters of light spots 25a, 25b, and 25c larger and increase jitters in a reproduced signal.